Basic Principles of Digital Modulation - Digital Video and Audio Broadcasting Technology - page 213

Mixers are today often implemented in the form of purely digital multi-
pliers which, except for quantization noise and rounding errors, have an
ideal behaviour.
If a direct voltage is applied as modulation signal, the carrier itself ap-
pears at the output of the mixer. Superimposing a sinusoidal signal on the
DC leads to normal amplitude modulation with unsuppressed carrier. (Fig.
13.4.).
Fig. 13.4. “Normal“ amplitude modulation with unsuppressed carrier
13.3 Amplitude Modulator
In amplitude modulation, the information is contained in the amplitude of
the carrier. The modulation signal changes (modulates) the carrier ampli-
tude. This is effected by means of an AM modulator.
Fig. 13.4. illustrates "normal" AM modulation, in which the carrier is
not suppressed. A sinusoidal modulation signal varies the carrier amplitude
and so is impressed on the carrier as an envelope. In the example in Fig.
13.4., both the carrier and the modulation signal are sinusoidal signals.
Looking at the spectrum, we not only find a spectral line at the carrier fre-